Method for reversing the direction of rotation of a two-stroke engine

ABSTRACT

A method for reversing the direction of rotation of a two-stroke engine whose rotational speed and crank mechanism position are sensed by a sensor system. To reverse the direction of rotation of the engine, the ignition and/or the fuel supply is first switched off, and upon a subsequent coasting of the engine, a targeted early ignition is set when a specific limiting rotational speed is undershot and after, if appropriate, the fuel supply has been resumed. Early ignition reverses the direction of rotation of the engine, and the fuel supply and ignition are subsequently controlled in accordance with the reversed direction of rotation. A single sensor interacts with an incremental transducer having a specific number of transducer segments distributed uniformly over a circumference to determine the instantaneous angular speed of the crank mechanism and the crank mechanism position.

FIELD AND BACKGROUND OF THE INVENTION

The invention is concerned with a method for reversing the direction ofrotation of a two-stroke engine whose rotational speed and crankmechanism position are sensed using a suitable sensor system, in which,in order to reverse the direction of rotation the ignition and/or thefuel supply is first switched off and when the engine subsequentlycoasts a targeted early ignition is set when a specific limitingrotation speed is undershot and after, if appropriate, the fuel supplyhas been resumed; which early ignition reverses the direction ofrotation of the engine, and the ignition and fuel supply aresubsequently controlled in accordance with the reversed direction ofrotation.

Such a method for reversing the direction of rotation of a two-strokeengine while operating is already known. The determination of thedirection of rotation which is necessary for the control process iscarried out here with at least two Hall sensors, in which case it ispossible not only to determine the rotational speed but also thedirection of rotation and position of the crankshaft from thechronological sequence of the sensor signals. In addition to theincreased costs for the second sensor, the costs for the mounting of asecond sensor, which also has to be connected to a control logic, arealso increased.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for reversingthe direction of rotation of a two-stroke engine which operates withonly one sensor.

The object is achieved according to the invention by means of a methodof the type described at the beginning in which a single sensorinteracts with an incremental transducer with a specific number oftransducer segments which are distributed uniformly over acircumference, and with a gap, and the instantaneous angular speed ofthe crank mechanism over the circumference is determined using thetransducer segments and the gap is evaluated in order to determine theabsolute crank mechanism position, in which method, when the enginecoasts, the fluctuations in the angular speed of the crank mechanismwhich are caused by the compression and expansion phases of at least onecombustion chamber of the engine are sensed during one rotation and areassigned to a specific transducer segment, and the direction of rotationis determined from the relative angular position of these transducersegments with respect to the gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, diagrammatically, a system for operation of an engine inaccordance with the invention;

FIG. 2 is a flow chart of operation of the engine;

FIG. 3 shows the flow chart of FIG. 2 with a further feature ofresynchronizing ignition and injection;

FIG. 4 shows a modification of the flow chart of FIG. 3; and

FIG. 5 is a mechanical diagram presenting components of an engineoperated in the system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The method utilizes the fluctuations in the angular speed which occur inan engine 10 (FIG. 5) which is coasting without ignition and/or fuelsupply and which result from the braking of the crank mechanism 12during the compression phases and a slight re-acceleration during theexpansion phases. It is possible to sense these fluctuations by using anincremental transducer 20 whose transducer segments 22 are provided witha specific angular spacing so that the precise angular speed can bedetermined repeatedly during one rotation from the chronologicalinterval between the triggered signals over the circumference. Duringthe coasting, there are, for example, local maximum values and minimumvalues in the angular speed which can be assigned to specific transducersegments 22, for example by counting the pulses since the gap of theincremental transducer 20 was last passed. As a result, in addition tothe gap, further information is obtained about the angular position ofthe crank mechanism 12 the assigned transducer element being a differentnumber of segments away from the gap depending on the direction ofrotation of the engine 10. The type of the sensor 14 which is used isirrelevant here, it is possible to use both inductive sensors and Hallsensors which interact, for example, with teeth as transducer segments22, or else it is also possible to use other sensors, for exampleoptically acting sensors which interact with a perforated disk or thelike as an incremental transducer, the gap being a closed hole in thiscase.

After a reversal of the direction of rotation has taken place, theposition of the ignition times and, if appropriate, injection times ofthe fuel supply 18 is preferably resynchronized (bottom of FIGS. 3, 4)with the gap 24 of the incremental transducer 20. This may he expedientin order to compensate positioning errors which may possibly occurduring the reversal of the direction of rotation since the number ofpulses which are triggered by the transducer segments 22 may fluctuatein the forward direction depending on the number of transducer segmentsstill passed after the early ignition 16 owing to the mass inertia ofthe crank mechanism 12. As a rule, the synchronization can be alreadyperformed during the first rotation in the opposite direction.

Furthermore, the method can be developed in such a way that after theearly ignition, a rise in the rotational speed of component 26 isanticipated after a number of sensor signals (sensor 14), the enginebeing switched off if said rise fails to occur. When the reversal of thedirection of rotation is successful, the engine is already stronglyaccelerated during its first rotation in the new direction of rotation,which can be sensed by the sensor 14 using the incremental transducer20. This rise in the rotational speed which fails to occur when the topdead center is incorrectly exceeded after the early ignition 16 in theprevious direction of rotation can be used as a signal to switch off theengine in order to avoid the situation in which the engine 10 which isstill running forward is affected by a completely incorrect ignitiontime during the subsequent rotation.

The method is particularly preferably used in a two-cylinder enginewhose cylinders are arranged offset by 180° on the crank mechanism, theassignment, i.e. the actuation of the ignition 16 and, if appropriate ofthe fuel injection 18, being interchanged between the first and secondcylinders. It would also be conceivable to change the actuation of thecylinders computationally, but the interchanging provides the particularadvantage that in the reverse direction of rotation the gap 24 of theincremental transducer which is usually arranged approximately 90°before the top dead center of the first cylinder with respect to theforward direction of rotation is also at a relatively small angulardistance with respect to the ignition process which is then actuallytaking place in the second cylinder, resulting in advantages for theactuation. In a single-cylinder engine, after the reversal of thedirection of rotation it is of course necessary to set the ignition and,if appropriate, the fuel injection to a changed relationship withrespect to the position of the gap of the incremental transducer. Undercertain circumstances it would also be conceivable to provide a furthergap which must not however make it more difficult to determine theposition of the crank mechanism in the way previously described.

In multi-cylinder engines, it is possible, depending on the offset ofthe right-angled bends of the crankshaft 26 which are assigned to thecylinders to change the new assignment after the reversal of thedirection of rotation by interchanging the cylinders in pairs or, forexample in the case of a three-cylinder engine, the assignment isre-determined with respect to the gap 24 of the incremental transducer.

A further advantage of the incremental transducer 20 is used in onepreferred development of the method according to which after the earlyignition is output, the number of transducer segments of the incrementaltransducer which match the sensor is counted, and when a specificlimiting number is exceeded the engine is switched off. This measurewhich is possible as an alternative or as a supplement to sensing therise in the rotational speed can also be used for the evaluation todetermine whether the reversal of the direction of the rotation of theengine was successful. The gap for the incremental transducer is, asalready mentioned generally up to approximately 90° before the top deadcenter of the piston with respect to the forward direction of rotation.The early ignition is, for example, preferably approximately 50° beforethe top dead center, i.e. approximately 4 to 5 transducer segments afterthe gap of the incremental transducer. If the direction of rotation issuccessfully reversed, the sensor senses considerably fewer segments upto the time when the gap is reached again, even when there areovershoots owing to the mass inertia of the crank mechanism, than whenthe top dead center is passed with a subsequent 270° rotation of theincremental transducer. If the gap is sensed after a number of sensorpulses in a number approximately equal to a quarter of the overallnumber of transducer segments, it is possible to assume that asuccessful reversal of the direction of rotation has occurred.

The subject-matter of the present invention is also a sensor systemwhich permits positions to be sensed using an incremental transducerhaving transducer segments distributed uniformly over the circumference,and using a sensor. According to the invention, a control logic 28(FIG. 1) senses cyclical fluctuations in the sensed angular speed duringone rotation which are caused by the compression and expansion phases ofthe at least one combustion chamber when the engine coasts in anon-driven fashion, and generates information about the angular positionof the crank mechanism by assignment of these fluctuations to specifictransducer segments of the incremental transducer. It is possible to usesuch a sensor system, which is suitable for use in various areas, todetermine the angular position of a crank mechanism. For the preferreduse in a method of the type described above, the incremental transducer20 preferably has a gap 24 which is preferably formed by shortening orcutting out two transducer segments which provide, in conjunction withthe sensor, further information about the angular position of the crankmechanism. By means of the assignment of specific gradients of theangular speed to specific transducer segments and by means of thesensing of the gap in addition to the determining of the instantaneousrotational speed and of the determining of the crank mechanism position,such a sensor system also permits the direction of rotation to bedetermined precisely since the angular spacing between the transducersegment which is determined and the gap is different depending on thedirection of rotation when the two-stroke engine coasts.

As already mentioned, the gap is preferably provided at 90° before thefirst or single cylinder of the engine, as viewed in the forwarddirection of rotation of the engine, in order, on the one hand, topermit appropriately timed triggering of the ignition pulses duringnormal running and, on the other hand, also to be able to trigger theearly ignition, preferably approximately 50° before the top dead center,even when there is a desired reversal of the direction of rotation.

The incremental transducers which have been used hitherto in the fieldof vehicles generally have a division into 60 teeth over thecircumference, the gap usually taking the place of two teeth for it tobe able to be sensed with certainty. In contrast, the sensor systemaccording to the invention preferably has only 36 transducer segments,preferably teeth, which are distributed over the circumference, two ofwhich are omitted in order to form the gap. This number of transducersegments has proven advantageous with an arrangement on the crankshaftof a two-stroke engine since, given a customary number of 60 teeth, anexcessively high interrupt load of the system results owing to the highrepetition rate at high rotational speeds. The number of approximately36 transducer segments has proven particularly advantageous here withrespect, on the one hand, to reducing the interrupt loading of thesystem and, on the other hand, to achieving sufficient resolution overthe circumference of the crank mechanism.

1. A method for reversing the direction of rotation of a two-strokeengine whose rotational speed and crank mechanism position are sensedusing a suitable sensor system, in which method, in order to reverse thedirection of rotation of the engine, the ignition and/or the fuel supplyis first switched off, and upon a subsequent coasting of the engine, atargeted early ignition is set when a specific limiting rotational speedis undershot and after, if appropriate, the fuel supply has beenresumed; wherein early ignition reverses the direction of rotation ofthe engine, and the fuel supply and ignition are subsequently controlledin accordance with the reversed direction of rotation; wherein a singlesensor interacts with an incremental transducer having a specific numberof transducer segments distributed uniformly over a circumference, andthe incremental transducer has a gap; and the instantaneous angularspeed of the crank mechanism over the circumference is determined usingthe transducer segments and the gap in order to determine the crankmechanism position; in which method, when the engine coasts,fluctuations in the angular speed of the crank mechanism, whichfluctuations are caused by compression and expansion phases of at leastone combustion chamber of the engine, are sensed during one rotation ofthe engine and are assigned to a specific transducer segment, and thedirection of rotation of the engine is determined from the relativeangular position of this transducer segment with respect to the gap. 2.The method as claimed in claim 1, wherein, after a reversal of thedirection of rotation, the position of the ignition times and, ifappropriate, injection times are resynchronized with the gap of theincremental transducer.
 3. The method as claimed in claim 1, wherein,after the reversal of the direction of rotation, a rise in therotational speed is anticipated after a number of sensor signals, theengine being switched off if said rise fails to occur.
 4. The method asclaimed in claim 1, wherein the engine is a two-cylinder engine withcylinders which are offset 180° on the crank mechanism, and anassignment between the first and second cylinders is interchanged aftera reversal of the direction of rotation.
 5. The method as claimed inclaim 1, wherein, in an engine with more than two cylinders, anassignment between cylinders which are arranged offset with respect toone another by 180° on the crank mechanism is interchanged in pairs, orwhen the offset of the cylinders deviates, the assignment ispredetermined in accordance with the offset with respect to the gap. 6.The method as claimed in claim 1, wherein, in a single-cylinder engine,in accordance with the position of the gap, an assignment in accordancewith the position of the gap, with respect to the upper dead center ofthe piston, is delayed by control means after the reversal of thedirection of rotation.
 7. The method as claimed in claim 1, wherein,after the early ignition is output, the number of transducer segments ofthe incremental transducer which match the sensor is counted, and when aspecific limiting number is exceeded the engine is switched off.
 8. Asensor system, comprising: a sensor, a control logic, and an incrementaltransducer having transducer segments on a rotating component of atwo-stroke engine, which transducer segments are distributed uniformlyover the circumference; wherein the incremental transducer has a gapwhich provides information about an angular position of a crankmechanism of the engine, and the control logic determines, by use of thetransducer segments, the instantaneous angular speed of the crankmechanism over the circumference, and senses, by use of the incrementaltransducer, cyclical fluctuations in the angular speed during onerotation of the engine, which fluctuations are caused by compression andexpansion phases of at least one combustion chamber of the engine whenthe engine coasts; and wherein the control logic generates informationabout the angular position of the crank mechanism by assignment tospecific transducer segments of the incremental transducer, anddetermines the direction of rotation of the engine by counting, betweenthe gap and the computationally determined crank mechanism position,control signals which are triggered by the transducer segments.
 9. Thesensor system as claimed in claim 8, wherein the gap is provided 90°before the first or single piston of the engine, viewed in a forwardrunning direction of the engine.
 10. The sensor system as claimed, inclaim 8, wherein the incremental transducer is composed of 36 transducersegments, two of which are shortened or cut away to form the gap. 11.The sensor system as claimed in claim 8, wherein the sensor is aninductive sensor.
 12. The sensor system as claimed claim 8, wherein thesensor is a Hall sensor.